Optical systems incorporating various combinations of optical elements, i.e. lenses, gratings and mirrors, may be subject to problems resulting from optical aberrations, such as coma, astigmatism, or spherical aberration. Certain configurations of optical systems having lens-mirror combinations (e.g. monochromators or spectrographs as may be used in ultraviolet light absorbance detectors ("UV detectors"), fluorescence detectors, High Performance Liquid Chromatography (HPLC) apparatus applications or the like), suffer performance problems resulting from such optical aberrations. Such performance problems include reduced optical throughput and reduced detector signal-to-noise ratio manifested as an increase in baseline absorbance noise.
Optical implementations described in U.S. Pat. Nos. 4,932,768 and 5,089,915 to Gobeli ("the '768 and '915 patents"), seek to correct spherical aberration and other optical aberrations by applying stresses to spherical mirrors in the optical system. The '768 and '915 patents disclose an optical system comprised of a toric mirror having a first element fabricated to possess a first rigidity factor, and a second element fabricated to possess a second rigidity factor higher than that of the first element. The second element includes a front surface sized to mate with the rear surface of the first element, along an interface zone. Interface contour means produces a differential cylindrical contour within the interface zone between the first and second elements. The interface contour means is aligned to modify the X-axis radius of curvature of the spherical reflector and compression means compresses the first and second elements together to deflect the first element relative to the second element, without substantially modifying the Y-axis radius of curvature.
The first element in the '768 and '915 patents is a spherical reflector formed on an aluminum substrate. A rear surface of the substrate is flat and has a series of three drilled and tapped one quarter inch deep apertures formed at intervals along a Y-axis of the substrate. The second element also includes a series of three spaced apart apertures dimensioned to precisely mate with the apertures in the first element. Interface contour means, in the form of a plurality of specially configured shims, are disposed in the interface zone between the first and second elements, and a plurality of threaded screws engage the apertures of the first and second elements to effect compression therebetween.
Generally, the implementation in the '768 and '915 patents applies stresses to spherical mirrors in one plane in Czerny-Turner spectrometer and spectrograph optical systems to improve the quality of an image in an exit focal plane. In one embodiment, the curvature of the original sphere of the mirrors in the two mirror Czerny-Turner configuration is modified by the applied stress in one plane, but the curvature is not altered in the plane at right angles thereto. The shape of the bend resulting from the applied stress is precisely controlled with multiple specially dimensioned shims. The magnitude of the applied stress is a function of specific relationships between the specially dimensioned shims and the compression applied via the particularly located screws. Careful consideration is taken in the '768 and '915 patents to ensure that curvature is altered only in a specific mirror axis orientation, while curvature in other axis orientations is avoided.
The implementation for correcting optical aberrations as described in the '768 and '915 patents is significantly limited in applicability, in that a toric mirror is required that is fabricated from at least two discrete elements having different respective rigidity factors. The elements must be configured to form an interface zone therebetween for receiving interface contour means.
Further, the implementation in the '768 and '915 patents is limited to the two mirror Czerny-Turner configuration, and seems more concerned with reducing coma and spherical aberration than astigmatism. Indeed, the preferred embodiment described in these patents requires that the first spherical mirror be bent in the plane which actually increases astigmatism, while the second mirror is bent in the plane at right angles. It is not clear that astigmatism is eliminated in this arrangement.
Still further disadvantages are associated with the implementation described in the '768 and '915 patents in that the basic spherical mirror as disclosed is replicated onto a metal substrate in order to achieve the appropriate relative rigidity factor(s), and to facilitate threading of screws into the mirror substrate. Such an implementation is much more expensive than fabricating a mirror by conventional grinding and polishing. Furthermore, the precise relationships between the mirror elements, interface zone and compression means leads to a highly complex configuration requiring various highly sophisticated parametric considerations.
A number of configurations have been used to try to improve the basic Czerny-Turner dual mirror configuration such as described in the '768 and '915 patents. Generally, the relative complexity and high parts count mitigates against the use of such configurations in relatively low resolution applications where low cost is a primary consideration. Nonetheless, efforts have been made to eliminate optical aberrations in these dual mirror systems, for example, by substituting off-axis parabolic mirrors for the spherical mirrors, to collimate the light onto the grating and focus the dispersed (collimated) light onto the exit slit. These enhancements tend to result in very expensive optical systems.
Other efforts to eliminate optical aberrations, and more specifically astigmatism, in optical systems such as monochromators and spectrometers have included the use of a weak cylindrical lens ahead of the entrance slit which can introduce a compensating astigmatism. Such an implementation requires that the additional lens be coupled with an over-tall entrance slit so that sagittal (S-image) and tangential (T-image) images of the light source can be brought together at a flowcell. This introduces at least one extra optical component, with associated cost and complexity. Furthermore, this implementation may introduce other image defects.
Conventional Ebert and Czerny-Turner monochromators involve two reflections from one or two spherical mirrors, respectively. Generally, when this arrangement is symmetrical, coma and spherical aberration introduced by the first off-axis reflection is compensated for by the second off-axis reflection. However, astigmatism at each off-axis reflection is additive. Prior art implementations for correcting such optical aberrations are overly complicated, costly and/or ineffectual.